Feed compression method and apparatus for air separation process

ABSTRACT

A method and apparatus for separating air to produce a gaseous oxygen product in which the air is separated in an air separation plant to conduct a cryogenic rectification process that utilizes higher and lower pressure compressed air streams. The higher and lower pressure compressed air streams are generated in two multistage compressors linked together so that the lower pressure compressed air stream is produced from intermediate stages and the higher pressure compressed air stream is produced from higher pressure compression stages. During turn-down operational conditions, one of the two multistage compressors can be shut down to decrease the flow of air and therefore, the production of the oxygen product.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for separatingair for producing an oxygen product in which the air is separated bycryogenic rectification within an air separation plant that utilizeshigher and lower pressure air streams. More particularly, the presentinvention relates to such a method and apparatus in which the air iscompressed in two multistage compressors that are linked together toproduce the higher pressure air stream from higher pressure stages andthe lower pressure air stream from lower pressure intermediate stagesand where one of the two multistage compressors can be turned off duringturn-down operational conditions to decrease the flow of compressed airto the plant and therefore, production of the oxygen product.

BACKGROUND OF THE INVENTION

A recent development in the field of air separation is the requirementto produce large quantities of low purity oxygen for use in thecombustion of coal in connection with electrical power generation andcarbon capture. Such applications can require as much as 10,000 tons perday of oxygen that is supplied by two or more air separation plants. Inorder for such power plants to be practical, it is necessary that thepower consumed by the air separation plants is as low as possible.Furthermore, since power plants can commonly operate at fractionalloads, it is also important for the air separation plants that are usedin connection with such power plants to also have a turn-down capabilityin which the air separation plant is able to efficiently supply the lowpurity oxygen by having a low power consumption during times that thepower plant is operating at less than full capacity.

Typically, an air separation plant can operate efficiently at no lowerthan 75 to 80 percent of fully capacity. Below this the powerconsumption of the air separation unit is nearly constant because thepower expended in compressing the air for the air separation plant hasnot changed, even where the air flow to the plant has been substantiallydecreased due to compressor surge limitations. Therefore, the problem inefficient turn-down operation for the air separation plant centers onthe compression equipment that is necessary to compress the air for theplant.

U.S. Pat. No. 4,895,583 discloses an air separation plant of the typethat uses both higher and lower pressure compressed air stream and thatis designed to produce low purity oxygen. The air is compressed andpurified in two compressors and associated purification units to producea higher pressure compressed and purified air stream and a lowerpressure compressed and purified air stream. The higher pressurecompressed and purified air stream is introduced into a first separationzone contained in a higher pressure column of the distillation columnsystem for rectification. The resulting kettle liquid produced in thefirst separation zone is subcooled and introduced into a secondseparation zone formed in an upper section of the lower pressure columnfor further refinement. A lower portion of the second separation zone,situated at an intermediate location of the lower pressure column, isreboiled by removing a liquid stream at such location that constitutes aportion of the down coming liquid and vaporizing such stream throughindirect heat exchange with a nitrogen-rich vapor produced as columnoverhead in the higher pressure column. The vaporized stream is returnedto the lower pressure column to a stage below that at which the liquidstream is withdrawn. The resulting condensed nitrogen-rich vapor is usedto reflux the first and second separation zones contained in the higherpressure column and the lower pressure column. An oxygen-rich columnbottoms liquid collected in a third separation zone located in a lowerstripping section of the lower pressure column. The bottoms liquid isvaporized through indirect heat exchange with the lower pressurecompressed and purified air stream in a condenser to produce liquid airand an oxygen-rich vapor that is in part returned to the lower sectionof the lower pressure column and in part warmed and taken as the oxygenproduct. The liquid air is introduced into the higher pressure columnand the lower pressure column as intermediate reflux.

U.S. Pat. No. 5,337,570 discloses an air separation process also havingthree separation zones contained in higher and lower pressure columns.In this patent, the air is compressed and purified to produce a higherpressure compressed and purified air stream and a lower pressurecompressed and purified air stream. The lower pressure compressed andpurified air stream is introduced into a first separation zone locatedin a higher pressure column for rectification to produce a kettle liquidas a column bottoms and a nitrogen-rich vapor as a column overhead. Thenitrogen-rich vapor is condensed through indirect heat exchange with akettle liquid stream composed of the kettle liquid. The kettle liquid ispartially vaporized to produce liquid and vapor phases that areintroduced into a bottom region of a second separation zone located inthe a lower pressure column for further refinement and the condensednitrogen-rich vapor is used to reflux the first and second separationzones located in the higher pressure column and an upper section of thelower pressure column. Part of the higher pressure compressed andpurified air stream reboils a third separation zone located in a lowersection of the lower pressure column to produce liquid air that is usedto form part of an intermediate reflux for the first separation zone andthe second separation zone situated in the higher pressure column andthe lower pressure column. Another part of the higher pressurecompressed and purified air stream is further compressed and used tovaporize an oxygen-rich liquid removed from the third separation zone,after pumping, to produce the oxygen product. The resulting liquid airis combined with the liquid air produced as a result of reboiling thelower section of the lower pressure column is also used in forming theintermediate reflux.

As will be discussed, the present invention provides a method andapparatus for separating air that, among other advantages, incorporatesa compression system that allows for a greater operational efficiencyduring turndown of compressors used in compressing the air than in theprior art.

SUMMARY OF THE INVENTION

The present invention provides a method of separating air to produce agaseous oxygen product. In accordance with such method, the air isseparated in a cryogenic rectification process that is configured toproduce an oxygen-rich stream by cryogenically rectifying the air withina distillation column system and warming the oxygen-rich stream, therebyto produce the gaseous oxygen product. The cryogenic rectificationprocess utilizes a first compressed air stream and a second compressedair stream having a lower pressure than the first compressed air stream.The first compressed air stream and the second compressed air stream areproduced to feed the cryogenic rectification process by compressing theair in two multistage compressors linked together such that during anormal mode of operation, a higher pressure air stream is produced fromhigher pressure stages of the two multistage compressors and a lowerpressure air stream is produced from lower pressure intermediate stagesof the two multistage compressors. The heat of compression is removedfrom the higher pressure air stream and the lower pressure air streamand the lower pressure air stream and the higher pressure air stream arepurified to produce a higher pressure compressed and purified air streamand a lower pressure compressed and purified air stream. The higherpressure air stream is formed from at least part of the higher pressurecompressed and purified air stream and the lower pressure air stream isformed from at least part of the lower pressure compressed and purifiedair stream. The cryogenic rectification process is able to be operatedin a turn-down mode of operation by turning off one of the twomultistage compressors and producing the higher pressure air stream andthe lower pressure air stream from higher and lower pressureintermediate stages, respectively, of the other of the two multistagecompressors, thereby decreasing air flow to the cryogenic rectificationprocess and therefore, production of the oxygen-rich stream and theoxygen product.

The first compressed air stream can be formed from the lower pressurecompressed and purified air stream and the second compressed air streamcan be formed from a first part of the higher pressure compressed andpurified air stream. A second part of the higher pressure compressed andpurified air stream is further separately compressed in a boostercompressor to produce a boosted pressure air stream that is partiallycooled and then expanded to produce an exhaust stream. The exhauststream is introduced into the distillation column system to impartrefrigeration into the cryogenic rectification process.

The air contained in the first compressed air stream can be rectified ina first separation zone of a distillation column system to produce akettle liquid and a nitrogen-rich vapor. A kettle liquid stream composedof the kettle liquid is introduced into a second separation zone of thedistillation column system for further refinement. The second separationzone operates at a lower operational pressure than the first separationzone. Down coming liquid entering a bottom region of the secondseparation zone is partly vaporized through indirect heat transfer witha nitrogen-rich vapor stream composed of the nitrogen-rich vaporproduced in the first separation zone, thereby condensing thenitrogen-rich vapor stream and forming reflux for the first separationzone and the second separation zone and a crude oxygen liquid fromresidual liquid not vaporized through the indirect heat exchange. Acrude oxygen liquid stream composed of the crude oxygen liquid isstripped in a third separation zone of the distillation column systemoperating at the lower operational pressure such that a nitrogencontaining vapor and an oxygen-rich liquid are produced. The oxygen-richliquid has a lower nitrogen content than the crude oxygen liquid. Anitrogen containing vapor stream composed of the nitrogen containingvapor is introduced into the second separation zone and the thirdseparation zone is reboiled with the second compressed air stream toform a liquid air stream. Intermediate reflux streams, composed at leastin part from the liquid air stream, are introduced into the firstseparation zone and the second separation zone and the oxygen-richstream is withdrawn from the third separation zone and is composed ofthe oxygen-rich liquid.

In a specific embodiment of the present invention, a third part of thehigher pressure compressed and purified air stream can be divided intotwo subsidiary streams that during the normal mode of operation arefurther separately compressed within two additional booster compressorsand recombined to produce a further compressed third part of thecompressed and purified air stream and during the turn-down mode ofoperation, the third part of the higher pressure compressed and purifiedair stream is further compressed within one of the two additionalbooster compressors to form the further compressed third part of thecompressed and purified air stream with the other of the two additionalbooster compressors turned off. The further compressed third part of thecompressed and purified air stream is fully cooled and the oxygen-richstream is vaporized and warmed in part through indirect heat exchangewith the further compressed third part of the compressed and purifiedair stream, thereby producing further liquid air. Intermediate reflux isintroduced in the first separation zone and the second separation zoneby subcooling a further liquid air stream composed of the further liquidair and a liquid air stream, composed of the liquid air produced inreboiling the third separation zone, through indirect heat exchange witha waste nitrogen stream produced in the second separation zone prior tothe waste nitrogen stream being fully warmed, part of the further liquidair stream is valve expanded and combined with the liquid air stream toproduce a first intermediate reflux stream which is further valveexpanded and introduced into the second separation zone and introducinga second intermediate reflux stream that is introduced into the secondseparation zone. The kettle liquid stream is introduced into the secondseparation zone by subcooling the kettle liquid stream through indirectheat exchange with the waste nitrogen stream prior to being fullywarmed, valve expanding the kettle liquid stream and introducing thekettle liquid stream into the second separation zone. A first refluxstream composed of part of the condensed nitrogen-rich vapor refluxesthe first separation zone and a second reflux stream composed of anotherpart of the condensed nitrogen-rich vapor is subcooled through indirectheat exchange with the waste nitrogen stream, valve expanded andrefluxes the second separation zone. The oxygen-rich stream can bepumped prior to the oxygen-rich stream being vaporized and warmedthrough the indirect heat exchange with the further compressed thirdpart of the compressed and purified air stream.

In any embodiment of the present invention, the distillation columnsystem can have a higher pressure distillation column that houses thefirst separation zone, a lower pressure distillation column that housesthe second separation zone and a side stripping column that houses thethird separation zone. The kettle liquid stream is introduced into thesecond separation zone in the rectification column.

In another aspect of the present invention, an apparatus is provided forproducing a gaseous oxygen product. In accordance with such aspect ofthe present invention, an air separation plant is configured to producean oxygen-rich stream. The air separation plant is of the type thatutilizes a first compressed air stream and a second compressed airstream and has a main heat exchanger to cool the first compressed airstream and the second compressed air stream. A distillation columnsystem is connected to the main heat exchanger to rectify the aircontained in the first compressed air stream and the second compressedair stream, thereby to produce the oxygen-rich stream and to return theoxygen-rich stream to the main heat exchanger such that the oxygen-richstream is fully warmed to produce the gaseous oxygen product. Acompression system is connected to a purification system to produce thefirst compressed air stream and the second compressed air stream. Thecompression system has two multistage compressors linked together suchthat during a normal mode of operation, a higher pressure air stream isproduced from higher pressure stages of the two multistage compressorsand a lower pressure air stream is produced from lower pressureintermediate stages of the two multistage compressors and after-coolersconnected to the higher pressure stages and the lower pressureintermediate stages remove heat of compression from the higher pressureair stream and the lower pressure air stream. The purification system isconfigured to purify the lower pressure air stream and the higherpressure air stream to produce a higher pressure compressed and purifiedair stream and a lower pressure compressed and purified air stream. Thepurification system is connected to the main heat exchanger such thatthe first compressed air stream is formed from at least part of thehigher pressure compressed and purified air stream and the secondcompressed air stream is formed from at least part of the lower pressurecompressed and purified air stream. A control system is provided forcontrolling the compression system such that the air separation plant isable to be selectively operated in the normal mode of operation and in aturn-down mode of operation in which one of the two multistagecompressors is turned off and producing the higher pressure air streamand the lower pressure air stream from higher and lower pressureintermediate stages, respectively, of the other of the two multistagecompressors, thereby decreasing air flow to the cryogenic rectificationprocess and therefore, production of the oxygen-rich stream and theoxygen product.

The purification system can be connected to the main heat exchanger suchthat the first compressed air stream is formed from the lower pressurecompressed and purified air stream and the second compressed air streamis formed from a first part of the higher pressure compressed andpurified air stream. A booster compressor is connected to thepurification system such that a second part of the higher pressurecompressed and purified air stream is further compressed in the boostercompressor to produce a boosted pressure air stream. The boostercompressor is connected to the main heat exchanger and the main heatexchanger is configured such that the boosted pressure air stream ispartially cooled within the main heat exchanger. A turbo-expander ispositioned between the main heat exchanger and the distillation columnsystem such that the boosted pressure air stream, after having beenpartially cooled, is expanded to produce an exhaust stream and theexhaust stream is introduced into the distillation column system toimpart refrigeration into the cryogenic rectification process.

The distillation column system can be provided with a first separationzone, a second separation zone and a third separation zone where thefirst separation zone has a higher operational pressure than the secondseparation zone. The first separation zone is connected to the main heatexchanger so as to receive the first compressed air stream forrectification. The second separation zone connected to the firstseparation zone such that a kettle liquid stream composed of kettleliquid produced in the first separation zone is introduced into thesecond separation zone for further refinement. A condenser-reboiler isconnected to the second separation zone and the first separation zonesuch that a nitrogen-rich vapor stream produced in the first separationzone indirectly exchanges heat to all down-coming liquid flowing towardsa bottom region of the second separation zone, thereby condensing thenitrogen-rich vapor stream and partly vaporizing the down-coming liquidto produce boil-up in the second separation zone and a crude oxygenliquid collected in the bottom region of the second separation zone fromdown-coming liquid that is not vaporized. The first separation zone andthe second separation zone are in flow communication with the condenserreboiler such that reflux streams produced as a result of the condensingof the nitrogen-rich vapor stream are introduced into the firstseparation zone and the second separation zone. The third separationzone is connected to a bottom region of the second separation zone toreceive a crude oxygen liquid stream, composed of the crude oxygenliquid and to return a nitrogen containing vapor stream back to thebottom region of the second separation zone. The third separation zoneconfigured to strip the crude oxygen liquid, thereby to produce thenitrogen containing vapor stream and an oxygen-rich stream having alower nitrogen content than the crude oxygen liquid. A reboiler islocated in the third separation zone and in flow communication with themain heat exchanger so as to receive the second compressed air stream,thereby to reboil the third separation zone and to produce liquid air.The first separation zone and the third separation zone are in flowcommunication with the reboiler such that at least one intermediatereflux stream composed, at least in part, of the liquid air isintroduced into at least one of the first separation zone and the secondseparation zone as intermediate reflux and the main heat exchanger is inflow communication with the third separation zone such that theoxygen-rich stream warms to form the oxygen product.

Two additional booster compressors can be positioned between thepurification system and the main heat exchanger such that during thenormal mode of operation a third part of the higher pressure compressedand purified air stream is divided into two further subsidiary streamsthat are separately compressed and combined to form a further compressedthird part of the higher pressure compressed and purified air stream.The control system also controls the two additional booster compressorssuch that during the normal mode of operation both of the two additionalbooster compressors are in operation and in the turn-down mode ofoperation, the third part of the higher pressure compressed and purifiedair stream compressed within one of the two additional boostercompressors to form the further compressed third part of the higherpressure compressed and purified air stream and the other of the twoadditional booster compressors is turned off. The main heat exchanger isconfigured such that the further compressed third part of the higherpressure compressed and purified air stream fully cools within the mainheat exchanger and a vaporizer is connected between the main heatexchanger and the third separation zone such that the oxygen-rich streamis removed from the third separation zone as a liquid and vaporized andwarmed in part through indirect heat exchange with the furthercompressed third part of the compressed and purified air stream, therebyto produce further liquid air. At least one subcooling unit is connectedbetween the reboiler, the second separation zone and the vaporizer suchthat a further liquid air stream composed of the further liquid air anda liquid air stream, composed of the liquid air, are subcooled throughindirect heat exchange with a waste nitrogen stream produced in thesecond separation zone, the at least one subcooling unit also connectedto the main heat exchanger such that the waste nitrogen stream fullywarms within the main heat exchanger. The second separation zone isconnected to the at least one subcooling unit such that a part of thefurther liquid air stream combines with the liquid air stream to producethe intermediate reflux stream. The first separation zone and the secondseparation zone are in flow communication with the at least onesubcooling unit such that a first intermediate reflux stream composed ofpart of the intermediate reflux stream is introduced into the secondseparation zone and a second intermediate reflux stream composed of afurther part of the intermediate reflux stream is introduced into thefirst separation zone. The first separation zone is connected to thecondenser reboiler such that a first of the reflux streams is introducedinto the first separation zone and the at least one subcooling unit isalso connected to the higher pressure distillation column such that thekettle liquid stream subcools within the at least one subcooling unitthrough indirect heat exchange with the waste nitrogen stream and asecond of the reflux streams subcools within the at least one subcoolingunit through indirect heat exchange with the waste nitrogen stream. Thesecond separation zone is connected to the at least one subcooling unitsuch that the second separation zone is refluxed with the second of thereflux streams. Expansion valves are positioned between the at least onesubcooling unit and the second separation zone such that the part of thefurther liquid air stream is expanded prior to combining with the liquidair stream and the first intermediate reflux stream is expanded prior toentering the second separation zone, the second of the reflux streams isexpanded prior to entering the first separation zone and the kettleliquid stream is expanded before introduction into the second separationzone.

In any embodiment of an apparatus of the present invention, thedistillation column system can have a higher pressure distillationcolumn that houses the first separation zone, a lower pressuredistillation column that houses the second separation zone and a sidestripping column that houses the third separation zone. The bottomregion of the second separation zone is a sump in the lower pressuredistillation column that contains the condenser reboiler.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with clams distinctly pointing out thesubject matter that Applicant regards as his invention, it is believedthat the invention will be better understood when taken in connectionwith the accompanying drawings in which:

FIG. 1 is a schematic of an apparatus designed to carry out a method inaccordance with the present invention;

FIG. 2 is a schematic process flow diagram of an air separation plantthat can be used in the apparatus shown in FIG. 1;

FIG. 3 is an alternative embodiment of FIG. 2;

FIG. 4 is an alternative embodiment of FIG. 2; and

FIG. 5 is an alternative embodiment of FIG. 2.

DETAILED DESCRIPTION

With reference to FIG. 1, an apparatus 1 is illustrated that is aschematic of an air separation plant in accordance with the presentinvention. Apparatus 1 incorporates a compression system 2 to compressthe air, a purification system 3 to purify the air, a main heatexchanger and a distillation column system (designated by referencenumbers 62 and 64, respectively, in FIG. 2) that is schematicallygrouped together in a box 4.

Compression system 2 incorporates two multistage compressors 10 and 12to compress two feed air streams 14 and 16. Although not illustrated,each of the multistage compressors could incorporate interstage coolingwith condensate removal. Additionally, each of the multistagecompressors contain inlet guide vanes 11 and 13 to adjust the flow ofthe air into such compressors. The inlet guide vanes 11 and 13 arecapable of adjusting the flow of the two feed air streams 14 and 16 intothe two multistage compressors 10 and 12 to decrease the flow from 100percent down to a flow of between 70 percent and 80 percent. Where lessflow is desired, part of the compressed air is vented or recirculated.Although not illustrated, each of the multistage compressors 10 and 12could be provided with vent and recirculation lines to allow for suchoperation. Optionally, intermediate guide vanes may be incorporated intothe two multistage compressors 10 and 12 (not shown) for optimal controlof the lower pressure air streams 30 and 34 and higher pressure airstreams 18 and 20 flow ratios.

The two multistage compressors 10 and 12 are linked together such thattwo subsidiary higher pressure air streams 18 and 20 produced fromhigher pressure final stages 22 and 24, respectively, are able tocombine to produce a higher pressure air stream 26. Higher pressure airstream 26 passes through an aftercooler 28 to remove the heat ofcompression. Aftercooler 28 may include further air chilling means, suchas mechanical or absorption chilling. Two subsidiary lower pressure airstreams 30 and 32 are produced from lower pressure intermediatecompression stages 34 and 36 that combine to produce a lower pressureair stream 38. The heat of compression is removed from the lowerpressure air stream 38 by an aftercooler 40. Aftercooler 40 may includefurther air chilling means, such as mechanical or absorption chilling.

The higher pressure air stream 26 and the lower pressure air stream 38are then purified within a purification system 3 that contains separatepurification units 42 and 44 to produce a higher pressure compressed andpurified air stream 46 and a lower pressure compressed and purified airstream 48. As well know in the art, purification units 42 and 44 areemployed to remove higher boiling impurities such as water vapor, carbondioxide and hydrocarbons from the air. Such units can incorporateadsorbent beds operating in an out of phase cycle that can be acombination of temperature and pressure swing adsorption to adsorb thehigher boiling impurities in an on-line bed and to regenerate beds in anoff-line status to desorb that higher boiling impurities and return thebeds to an on-line status. Typically such regeneration will utilizewaste nitrogen produced from the air separation unit 4. Although notillustrated, the purification could be accomplished in a single unithaving three or more beds in which two beds are in operation and aretherefore, in an on-line status at any given time with the third bed inan off-line status being regenerated. Such a unit is described in detailin U.S. Pat. No. 5,337,570.

A first compressed air stream 50 is produced from a first part of thehigher pressure compressed and purified air stream 46. In theillustrated embodiment, all of the lower pressure compressed andpurified air stream 48 forms a second compressed air stream 52. Thesecond compressed air stream 52 and the first compressed air stream 50,having a higher pressure than the second compressed air stream 52, alongwith other air streams to be discussed, are cooled within the main heatexchanger and rectified within distillation column system to produce anoxygen product stream 54 and a waste nitrogen stream 56.

It is understood that the apparatus 1 can be of any design that useshigher and lower compressed air streams. As will be discussed, the firstcompressed air stream 50 is utilized in a specific embodiment describedbelow for reboiling duty, although this does not have to be the case.For example, the first compressed air stream 50 could be used in heatingan oxygen-rich liquid stream to produce an oxygen product stream.Alternatively, the first compressed air stream 50 could be partiallycooled, expanded and introduced into a higher pressure column forpurposes of imparting refrigeration. In specific embodiments of thepresent invention, to be discussed, the second compressed air stream 52after having been cooled is introduced into a higher pressure column forrectification. However, such second compressed air stream 52 could be ofa sufficiently low pressure that it would be introduced into a lowerpressure column. Additionally, not all of the lower pressure compressedand purified air stream 48 need be used in forming the second compressedstream 52. In this regard, part of the lower pressure compressed andpurified air stream could be used for other purposes, for example,turbine expansion and introduction into the low pressure column afterpartial cooling. Other elements shown in FIG. 1 will be discussed inmore detail in FIG. 2 with respect to the more detailed description ofan embodiment of the apparatus 1 and in particular, the main heatexchanger and the distillation column system.

The multistage compressors 10 and 12 of the compression system 2 arecontrolled by a controller 5 that are connected to the multistagecompressors 10 and 12 by electrical connections 58 and 60. Controller 5could be a primary and supervisory control system that is capable of atleast shutting off one of the two multistage compressors 10 and 12,controlling the inlet guide vanes 11 and 13 and again, although notillustrated, also controlling a vent lines and recirculation loops.During normal operating conditions both the higher pressure compressedand purified air stream 46 and the lower pressure compressed andpurified air stream 48 and therefore, the first compressed air stream 50and the second compressed air stream are formed by the two multistagecompressors 12 and 10 respectively. However, during turndown operatingconditions when a lower flow rate for the oxygen product stream 54 isdesired, the first compressed air stream 50 and the second compressedair stream 52 are formed by turning off compressor 10 and forming bothsuch streams, at a lower flow rate, from multistage compressor 12. Undersuch operational conditions, the multistage compressor 12 can to alimited extent be turned down by use of its inlet guide vanes 13 to evenprovide lower air flow. Typically, control of the flow by the inletguide vanes 11 and 13 allows the two multistage compressors 10 and 12 tobe turned down to between 70 and 80 percent of capacity. In fact, thepresent invention as set forth in the appended claims is not meant toexclude operation of both of the two multistage compressors 10 and 12 ata reduced capacity with inlet guide vane control. However, below thelevel of between 70 and 80 percent of capacity, aerodynamicconsiderations come into play in which the compressor cannot be turneddown without surging. In order to overcome this, vent and recirculationlines are provided to allow the output of the compressor to be furtherreduced. However, the venting or recirculation of the compressed air,while reducing the flow output of the compressor will not reduce thepower consumption of the compressor. Thus, during turndown conditions ofthe plant, assuming the most ideal compressor in which power consumptionis proportional to flow through the compressor, the power consumptionwill never be reduced below about 80 percent even when less than 80percent of the flow is desired. In the present invention, however, byemploying two multistage compressors, during turndown conditions one ofthe compressors is able to be turned off. This allows the compressionsystem 2 to consume 50 percent of the power that it would be required toconsume during normal operational conditions and with inlet guide vanes,the range of the compression system 2 will be from 40 to 100 percentwith the power consumption being proportional to the flow rate at flowoutput of 40 percent, 50 percent and 80 percent.

The compression system 2 can be one of a series of compression systemsemployed in an enclave of air separation plants. In such case, a greaterrange of oxygen production can be obtained by either shutting down oneof the plants or turning off one of the multistage compressors of aplant and etc.

With reference to FIG. 2, a process flow diagram of an embodiment of themain heat exchanger 62 and the distillation column system 64 containedin block 4 of FIG. 1 are illustrated. The first and second compressedair streams 50 and 52 are cooled within a main heat exchanger 62 that inpractice would consists of a series of such heat exchangers linked inparallel. The first compressed air stream 52 is fully cooled in the mainheat exchanger 62 to a temperature suitable for its rectification. Inthis regard, the term “fully cooled” as used herein and in the claimsmeans cooled to a temperature at the cold end of the main heat exchanger2. The first compressed air stream 52 is thereafter introduced into ahigher pressure distillation column 66 of the distillation column system64 that houses a first separation zone in which the air is rectifiedinto a kettle liquid 68 and a nitrogen-rich vapor column overhead. Masstransfer contacting elements 70 and 72 are provided within distillationcolumn 66 to contact liquid and vapor phases. The liquid phase becomesever more rich in oxygen as it descends within distillation column 66and the vapor phase becomes rich in nitrogen as it ascends withindistillation column 66 to produce the kettle liquid 68 and thenitrogen-rich vapor, respectively. As well known in the art, masstransfer contacting elements 18 and 20 can be structured packing ortrays or random packing or a combination of packing and trays.

A second separation zone is provided that is housed within a lowerpressure distillation column 74 of the distillation column system 64that is thermally linked to the first separation zone housed within thehigher pressure distillation column 66 with the use of a condenserreboiler 76 that is illustrated as located in a sump 78 of the lowerpressure distillation column 74 that forms the bottom region of thesecond separation zone. The condenser reboiler 76 could be locatedexternal to the lower pressure distillation column 74. A nitrogen-richvapor stream 80 composed of the nitrogen-vapor column overhead producedin the higher pressure column 66 is introduced into the condenserreboiler 76 where it is condensed through indirect heat exchange withdown coming liquid produced in the lower pressure column. All of suchdown coming liquid is contacted with the condenser reboiler 76 topartially vaporize such liquid and thereby to produce boil-up within thelower pressure distillation column 74. Residual liquid 82 that is notvaporized collects within the sump 78 as a crude oxygen liquid. Thecondensed nitrogen-rich vapor produces a nitrogen-rich liquid stream 84to produce reflux for the columns. In this regard, a first reflux stream86 composed of part of the nitrogen-rich liquid stream 84 refluxes thehigher pressure column and a second reflux stream 88 is introduced intoa subcooling unit 90, valve expanded by an expansion valve 92 and thenintroduced as reflux to the lower pressure column 74. The introductionof the second reflux stream 88 initiates the formation of a descendingliquid phase and the boil-up produced by the condenser reboiler 76, inpart, initiates an ascending liquid phase that are contacted by means ofmass transfer contacting elements 94, 95 and 96 within the lowerpressure column 74 that can be structured packing, random packing, traysor a combination of such elements. All of the down coming liquid is theliquid phase enriched in oxygen is discharged from the lowermost masstransfer contacting element 96 to the condenser reboiler 76. A kettleliquid stream 98, composed of the kettle liquid 68 is subcooled withinsubcooling unit 90 and is introduced into the lower pressure column 74after valve expansion within expansion valve 99.

The third separation zone is providing by a stripping column 100 of thedistillation column system 64 that contains mass transfer contactingelements 102 such as described above. A crude oxygen liquid stream 104that is composed of the crude oxygen liquid 82 is introduced into thestripping column 100 and is stripped to produce a nitrogen containingvapor that is returned to the sump 78 of the lower pressure distillationcolumn 74 as a nitrogen containing vapor stream 106. The strippingcolumn 100 is reboiled by the first compressed air stream 50 afterhaving been fully cooled within the main heat exchanger 62 byintroducing the first compressed air stream 50 into a reboiler 108 toproduce a liquid air stream 110 that, as will be discussed, forms partof the intermediate reflux that is supplied to the higher pressuredistillation column 66 and the lower pressure distillation column 74.The resulting residual liquid forms an oxygen-rich liquid 112 that canbe taken as an oxygen-rich stream 114 in forming the oxygen productstream 54. In this regard, oxygen-rich stream 114 is vaporized in avaporizer 116. Vaporizer 116 has a shell 118 and a heat exchanger 120enclosed within the shell 118. The resulting vaporized oxygen-richstream 122 is fully warmed within the main heat exchanger 62 to producean oxygen product stream 54.

The first compressed air stream 50 is of a sufficiently high pressureand flow rate to provide boil-up within stripping column 100 that actsto increase the compositional difference between the liquid and vaporphases that would otherwise have been obtained at the pressure of thesecond compressed air stream 52. This increases the nitrogen contentwithin the lower pressure column 74 and therefore decreases thetemperature within such column such that the down coming liquid fed tothe condenser reboiler 76 is able to condense the nitrogen-rich vapor ofthe nitrogen-rich vapor stream 80 at a lower pressure than the higherpressure of the first compressed air stream 50.

A second part 124 of the compressed and purified air stream 46 isintroduced into a booster compressor 126 to produce another boostedpressure air stream 128 that, after removal of the heat of compressionin an after cooler 130, is partially cooled in main heat exchanger 62and expanded within a turboexpander 132 to produce an exhaust stream134. As illustrated, the turboexpander 132 drives the booster compressor126. Exhaust stream 134 is then introduced into the lower pressurecolumn 74 to impart refrigeration into the apparatus 1.

A third part 136 of the higher pressure compressed and purified airstream 46 is divided into subsidiary streams 138 and 140 that, duringnormal operating conditions, are separately compressed in two boostercompressors 142 and 144 and recombined into a further boosted pressureair stream 146. After removal of the heat of compression from furtherboosted pressure air stream 146 in an after cooler 148, the furtherboosted pressure air stream 146 is fully cooled and then condensedwithin vaporizer 116 to form a further liquid air stream 150. Furtherliquid air stream 150 is then subcooled within subcooling unit 90 alongwith liquid air stream 110 and then combined with liquid air stream 110after valve expansion in an expansion valve 152. The combined liquid airstream is then divided into first and second intermediate reflux streams154 and 156, respectively. First and second intermediate reflux streams154 and 156 are then introduced as intermediate reflux into the lowerpressure column 74 and the higher pressure column 66 after expansionwithin expansion valves 158 and 160.

During a turndown mode of operation, controller 5 turns off boostercompressor 142 such that the further boosted pressure air stream 146 isformed through compression of the third part 136 of the higher pressurecompressed and purified air stream by booster compressor 144. For suchcontrol purposes, booster compressors 142 and 144 are connected tocontroller 5 by electrical conductors 162 and 164. As could beappreciated by those skilled in the art, booster compressor 142 and 144could be incorporated into the two multistage compressors 10 and 12 as ahigh pressure final stage that would be connected to and locateddownstream of the of the final stages 22 and 24 of the two multistagecompressors 10 and 12 illustrated in FIG. 1.

Subcooling duty for the subcooling unit 90 is provided by anitrogen-rich vapor stream 166 that is removed from the lower pressurecolumn 74. After partial warming within subcooling unit 166, thenitrogen-rich vapor stream 166 is fully warmed in the main heatexchanger 62 and discharged as the waste nitrogen stream 56.

With reference to FIG. 3, an alternative embodiment of the process flowdiagram of an embodiment of the main heat exchanger 62 and thedistillation column system 64 contained in block 4 of FIG. 1 isdesignated as 4′. This embodiment in most respects is the same as shownin FIG. 2 except that in place of the second reflux stream 86, a refluxstream 87 of lower purity is introduced into the lower pressure column74. For such purposes, the higher pressure distillation column isslightly modified and is illustrated as distillation column 66′ thatdiffers from distillation column 66 by the replacement of mass transfercontacting elements 72 with mass transfer contacting elements 73 and 75.Although not illustrated a liquid collector would be positioned betweensuch elements to permit the withdrawal of second reflux stream 87. Anadvantage of this is that a nitrogen liquid product stream 88′ in placeof the second reflux stream 88 shown in FIG. 2 can be withdrawn,subcooled within subcooling unit 90 and then used or sent to storage.

With reference to FIG. 4, an alternative embodiment of the process flowdiagram of an embodiment of the main heat exchanger 62 and thedistillation column system 64 contained in block 4 of FIG. 1 isdesignated as 4″. In this embodiment, the oxygen-rich liquid stream 114is pumped by a pump 170 to produce a pressurized oxygen-rich liquidstream 122′ that is heated in the main heat exchanger 62 to produce theoxygen product stream 54. If the pressurized oxygen-rich liquid stream122′ is supercritical, the resulting oxygen product stream 54 will be asupercritical fluid. In this regard, it is possible to bank the mainheat exchanger 62 into a lower pressure bank that would cool the streamspreviously discussed and a high pressure bank to heat the oxygen-richliquid stream 122′ through indirect heat exchange with further boostedpressure air stream 146. If, however, oxygen-rich liquid stream 122′ isat a lower pressure, below supercritical, the resulting oxygen productstream 54 would be a high pressure vapor product.

With reference to FIG. 5, a further alternative embodiment of theprocess flow diagram of an embodiment of the main heat exchanger 62 andthe distillation column system 64 contained in block 4 of FIG. 1 isdesignated as 4′″. In this embodiment, the distillation column system 64is replaced by a distillation column system 64′ having the second andthird separation zones that in the preceding embodiments were housed inthe lower pressure distillation column 74 and the stripping column 100into a distillation column 172 having a second separation zone 174 and athird separation zone 176. The bottom region of the second separationzone is designated by column section 78′. The oxygen rich liquid withinthe bottom regions of the second separation zone 174 would bedistributed to the third separation zone 176 by a liquid distributorthat, although not illustrated, would be conventional and includeopenings to allow the nitrogen containing vapor produced in the thirdseparation zone 176 to the second separation zone 174. The operation ofsuch embodiment is otherwise the same as that shown in FIG. 1.

Although the present invention has been described with reference topreferred embodiments, as will occur to those skilled in the art,numerous changes, additions and omissions can be made without departingfrom the spirit and scope of the present invention as set forth in theappended claims.

1. A method of separating air to produce a gaseous oxygen productcomprising: separating the air in a cryogenic rectification process thatis configured to produce an oxygen-rich stream by cryogenicallyrectifying the air within a distillation column system and warming theoxygen-rich stream, thereby to produce the gaseous oxygen product andthat utilizes a first compressed air stream and a second compressed airstream having a lower pressure than the first compressed air stream;producing the first compressed air stream and the second compressed airstream to feed the cryogenic rectification process by compressing theair in two multistage compressors linked together such that during anormal mode of operation, a higher pressure air stream is produced fromhigher pressure stages of the two multistage compressors and a lowerpressure air stream is produced from lower pressure intermediate stagesof the two multistage compressors, removing heat of compression from thehigher pressure air stream and the lower pressure air stream andpurifying the lower pressure air stream and the higher pressure airstream to produce a higher pressure compressed and purified air streamand a lower pressure compressed and purified air stream and forming thehigher pressure air stream from at least part of the higher pressurecompressed and purified air stream and the lower pressure air streamfrom at least part of the lower pressure compressed and purified airstream; and operating the cryogenic rectification process in a turn-downmode of operation by turning off one of the two multistage compressorsand producing the higher pressure air stream and the lower pressure airstream from higher and lower pressure intermediate stages, respectively,of the other of the two multistage compressors, thereby decreasing airflow to the cryogenic rectification process and therefore, production ofthe oxygen-rich stream and the oxygen product.
 2. The method of claim 1,wherein: the first compressed air stream is formed from the lowerpressure compressed and purified air stream and the second compressedair stream is formed from a first part of the higher pressure compressedand purified air stream; a second part of the higher pressure compressedand purified air stream is further separately compressed in a boostercompressor to produce a boosted pressure air stream; the boostedpressure air stream partially cooled and then expanded to produce anexhaust stream; and the exhaust stream is introduced into thedistillation column system to impart refrigeration into the cryogenicrectification process.
 3. The method of claim 2, wherein: the aircontained in the first compressed air stream is rectified in a firstseparation zone of a distillation column system to produce a kettleliquid and a nitrogen-rich vapor; a kettle liquid stream composed of thekettle liquid is introduced into a second separation zone of thedistillation column system for further refinement, the second separationzone operating at a lower operational pressure than the first separationzone; down coming liquid entering a bottom region of the secondseparation zone is partly vaporized through indirect heat transfer witha nitrogen-rich vapor stream composed of the nitrogen-rich vaporproduced in the first separation zone thereby condensing thenitrogen-rich vapor stream and forming reflux for the first separationzone and the second separation zone and a crude oxygen liquid fromresidual liquid not vaporized through the indirect heat exchange; acrude oxygen liquid stream composed of the crude oxygen liquid isstripped in a third separation zone of the distillation column systemoperating at the lower operational pressure such that a nitrogencontaining vapor and an oxygen-rich liquid are produced, the oxygen-richliquid having a lower nitrogen content than the crude oxygen liquid; anitrogen containing vapor stream composed of the nitrogen containingvapor is introduced into the second separation zone; the thirdseparation zone is reboiled with the second compressed air stream toform a liquid air stream; intermediate reflux streams, composed at leastin part from the liquid air stream, are introduced into the firstseparation zone and the second separation zone; and the oxygen-richstream is withdrawn from the third separation zone and is composed ofthe oxygen-rich liquid.
 4. The method of claim 3, wherein: a third partof the higher pressure compressed and purified air stream is dividedinto two further subsidiary streams that during the normal mode ofoperation are separately compressed within two additional boostercompressors and recombined to produce a further compressed third part ofthe compressed and purified air stream and during the turn-down mode ofoperation the third part of the higher pressure compressed and purifiedair stream is further compressed within one of the two additionalbooster compressors to form the further compressed third part of thecompressed and purified air stream with the other of the two additionalbooster compressors turned off; the further compressed third part of thecompressed and purified air stream is fully cooled; the oxygen-richstream is vaporized and warmed in part through indirect heat exchangewith the further compressed third part of the compressed and purifiedair stream, thereby producing further liquid air; and the intermediatereflux is introduced in the first separation zone and the secondseparation zone by subcooling a further liquid air stream composed ofthe further liquid air and a liquid air stream, composed of the liquidair produced in reboiling the third separation zone, through indirectheat exchange with a waste nitrogen stream produced in the secondseparation zone prior to the waste nitrogen stream being fully warmed,part of the further liquid air stream is valve expanded and combinedwith the liquid air stream to produce a first intermediate reflux streamwhich is further valve expanded and introduced into the secondseparation zone and introducing a second intermediate reflux stream thatis introduced into the first separation zone; the kettle liquid streamis introduced into the second separation zone by subcooling the kettleliquid stream through indirect heat exchange with the waste nitrogenstream prior to being fully warmed, valve expanding the kettle liquidstream and introducing the kettle liquid stream into the secondseparation zone; and a first reflux stream composed of part of thecondensed nitrogen-rich vapor refluxes the first separation zone and asecond reflux stream composed of another part of the condensednitrogen-rich vapor is subcooled through indirect heat exchange with thewaste nitrogen stream, valve expanded and refluxes the second separationzone.
 5. The method of claim 4, wherein the oxygen-rich stream is pumpedprior to the oxygen-rich stream being vaporized and warmed through theindirect heat exchange with the further compressed third part of thecompressed and purified air stream.
 6. The method of claim 3 or claim 4or claim 5, wherein: the distillation column system has a higherpressure distillation column that houses the first separation zone, alower pressure distillation column that houses the second separationzone and a side stripping column that houses the third separation zone;and the kettle liquid stream is introduced into the second separationzone in the rectification column.
 7. An apparatus for producing agaseous oxygen product comprising: an air separation plant configured toproduce an oxygen-rich stream and utilizing a first compressed airstream and a second compressed air stream, the air separation planthaving a main heat exchanger to cool the first compressed air stream andthe second compressed air stream, a distillation column system connectedto the main heat exchanger to rectify the air contained in the firstcompressed air stream and the second compressed air stream, thereby toproduce the oxygen-rich stream and to return the oxygen-rich stream tothe main heat exchanger such that the oxygen-rich stream is fully warmedto produce the gaseous oxygen product, a compression system connected toa purification system to produce the first compressed air stream and thesecond compressed air stream; the compression system having twomultistage compressors linked together such that during a normal mode ofoperation, a higher pressure air stream is produced from higher pressurestages of the two multistage compressors and a lower pressure air streamis produced from lower pressure intermediate stages of the twomultistage compressors and after-coolers connected to the higherpressure stages and the lower pressure intermediate stages for removingheat of compression from the higher pressure air stream and the lowerpressure air stream the purification system configured to purify thelower pressure air stream and the higher pressure air stream to producea higher pressure compressed and purified air stream and a lowerpressure compressed and purified air stream; the purification systemconnected to the main heat exchanger such that the first compressed airstream is formed from at least part of the higher pressure compressedand purified air stream and the second compressed air stream is formedfrom at least part of the lower pressure compressed and purified airstream; and a control system for controlling the compression system suchthat the air separation plant is able to be selectively operated in thenormal mode of operation and in a turn-down mode of operation in whichone of the two multistage compressors is turned off and producing thehigher pressure air stream and the lower pressure air stream from higherand lower pressure intermediate stages, respectively, of the other ofthe two multistage compressors, thereby decreasing air flow to thecryogenic rectification process and therefore, production of theoxygen-rich stream and the oxygen product.
 8. The apparatus of claim 7,wherein: the purification system is connected to the main heat exchangersuch that the first compressed air stream is formed from the lowerpressure compressed and purified air stream and the second compressedair stream is formed from a first part of the higher pressure compressedand purified air stream; a booster compressor is connected to thepurification system such that a second part of the higher pressurecompressed and purified air stream is further compressed in the boostercompressor to produce a boosted pressure air stream; the boostercompressor is connected to the main heat exchanger and the main heatexchanger is configured such that the boosted pressure air stream ispartially cooled within the main heat exchanger; a turbo-expander ispositioned between the main heat exchanger and the distillation columnsystem such that the boosted pressure air stream, after having beenpartially cooled, is expanded to produce an exhaust stream and theexhaust stream is introduced into the distillation column system toimpart refrigeration into the cryogenic rectification process.
 9. Theapparatus of claim 8, wherein: the distillation column system has afirst separation zone, a second separation zone and a third separationzone, the first separation zone having a higher operational pressurethan the second separation zone; the first separation zone connected tothe main heat exchanger so as to receive the first compressed air streamfor rectification; the second separation zone connected to the firstseparation zone such that a kettle liquid stream composed of kettleliquid produced in the first separation zone is introduced into thesecond separation zone for further refinement; a condenser-reboiler isconnected to the second separation zone and the first separation zonesuch that a nitrogen-rich vapor stream produced in the first separationzone indirectly exchanges heat to all down-coming liquid flowing towardsa bottom region of the second separation zone, thereby condensing thenitrogen-rich vapor stream and partly vaporizing the down-coming liquidto produce boil-up in the second separation zone and a crude oxygenliquid collected in the bottom region of the second separation zone fromdown-coming liquid that is not vaporized; the first separation zone andthe second separation zone are in flow communication with the condenserreboiler such that reflux streams produced as a result of the condensingof the nitrogen-rich vapor stream are introduced into the firstseparation zone and the second separation zone; the third separationzone is connected to a bottom region of the second separation zone toreceive a crude oxygen liquid stream, composed of the crude oxygenliquid and to return a nitrogen containing vapor stream back to thebottom region of the second separation zone, the third separation zoneconfigured to strip the crude oxygen liquid, thereby to produce thenitrogen containing vapor stream and an oxygen-rich stream having alower nitrogen content than the crude oxygen liquid; a reboiler islocated in the third separation zone and in flow communication with themain heat exchanger receives the second compressed air stream, therebyto reboil the third separation zone and to produce liquid air; the firstseparation zone and the third separation zone are in flow communicationwith the reboiler such that at least one intermediate reflux streamcomposed, at least in part, of the liquid air is introduced into atleast one of the first separation zone and the second separation zone asintermediate reflux; and the main heat exchanger is in flowcommunication with the third separation zone such that the oxygen-richstream warms to form the oxygen product.
 10. The method of claim 9,wherein: two additional booster compressors are positioned between thepurification system and the main heat exchanger such that during thenormal mode of operation a third part of the higher pressure compressedand purified air stream is divided into two further subsidiary streamsthat are separately compressed and combined to form a further compressedthird part of the higher pressure compressed and purified air stream;the control system also controls the two additional booster compressorssuch that during the normal mode of operation both of the two additionalbooster compressors are in operation and in the turn-down mode ofoperation, the third part of the higher pressure compressed and purifiedair stream compressed within one of the two additional boostercompressors to form the further compressed third part of the higherpressure compressed and purified air stream and the other of the twoadditional booster compressors is turned off; the main heat exchanger isconfigured such that the further compressed third part of the higherpressure compressed and purified air stream fully cools within the mainheat exchanger; a vaporizer is connected between the main heat exchangerand the third separation zone such that the oxygen-rich stream isremoved from the third separation zone as a liquid and vaporized andwarmed in part through indirect heat exchange with the furthercompressed third part of the compressed and purified air stream, therebyproducing further liquid air; at least one subcooling unit is connectedbetween the reboiler, the second separation zone and the vaporizer suchthat a further liquid air stream composed of the further liquid air anda liquid air stream, composed of the liquid air, are subcooled throughindirect heat exchange with a waste nitrogen stream produced in thesecond separation zone, the at least one subcooling unit also connectedto the main heat exchanger such that the waste nitrogen stream fullywarms within the main heat exchanger; the second separation zone isconnected to the at least one subcooling unit such that a part of thefurther liquid air stream combines with the liquid air stream to producethe intermediate reflux stream; the first separation zone and the secondseparation zone are in flow communication with the at least onesubcooling unit such that a first intermediate reflux stream composed ofpart of the intermediate reflux stream is introduced into the secondseparation zone and a second intermediate reflux stream composed of afurther part of the intermediate reflux stream is introduced into thefirst separation zone; the first separation zone is connected to thecondenser reboiler such that a first of the reflux streams is introducedinto the first separation zone; the at least one subcooling unit is alsoconnected to the higher pressure distillation column such that thekettle liquid stream subcools within the at least one subcooling unitthrough indirect heat exchange with the waste nitrogen stream, a secondof the reflux streams subcools within the at least one subcooling unitthrough indirect heat exchange with the waste nitrogen stream; thesecond separation zone is connected to the at least one subcooling unitsuch that the second separation zone is refluxed with the second of thereflux streams; and expansion valves are positioned between the at leastone subcooling unit and the second separation zone such that the part ofthe further liquid air stream is expanded prior to combining with theliquid air stream and the first intermediate reflux stream is expandedprior to entering the second separation zone, the second of the refluxstreams is expanded prior to entering the first separation zone and thekettle liquid stream is expanded before introduction into the secondseparation zone.
 11. The method of claim 9 or claim 10, wherein: thedistillation column system has a higher pressure distillation columnthat houses the first separation zone, a lower pressure distillationcolumn that houses the second separation zone and a side strippingcolumn that houses the third separation zone; and the bottom region ofthe second separation zone is a sump in the lower pressure distillationcolumn that contains the condenser reboiler.